Embryonic and post-embryonic development in insects :its structure, functions and modification in different orders of insects

1. Embryonic and post-embryonic
development in insects :its structure,
functions and modification in different
orders of insects

2. INTRODUCTION
• Embryology is the branch of biology which deals with the growth and development of an
embryo of an organism, commencing with the union of male and female gametes.
• Embryology includes the development of the fertilized egg and embryo and the growth of the
organ system.
• Embryogenesis is the process by which a single egg develops into a multicellular individual.
Embryonic development in insects begin when the gametes produced by females(oocytes)
fuse with gametes produced from males(sperm) through the process of fertilization to form a
zygote.
• The zygote the divides mitotically to produce all different cells that comprise the body of
nymph(exopterygote/hemimetabolous species) or larva(endopterygote/holometabolous
species), which will hatch from egg.
• Broadly embryogenesis can be divided into two categories.
• 1. Early embryonic development: - events taking place inside the egg.
• 2. Post embryonic development: -events taking place after hatching from egg

3. Early Embryonic Development Fertilization
• The production of male(sperm) and female(ovum) gametes is
commonly considered to be The first phase in insect development.
• The union of gametes(spermatozoon and ovum) is The second phase
of development, and creates a diploid zygote with the potential to
form an entire organism and this process occurs through the
fertilization of ovum with sperms.
• The sperm of most of insects remain alive for month to years within
the Spermatheca. Fertilization occurs when the eggs are about to be
laid and as each passes down the oviduct. The sperm enter into the
eggs through micropyle. Several sperms penetrate into the eggs and
fertilization is affected by one of the sperms to form zygote while the
rest sperms degenerate.

4. THE EGG
• First stage of development in all oviparous insects is egg.it is the inconspicuous
inexpensive and inactive stage.
• It contains nucleus, cytoplasm, yolk (protein, lipids and carbohydrates) surrounding
these components is the eggshell.
• EGG SHELL
• 1. Vitelline membrane (innermost)
• 2. Wax layer
• 3. Chorion (outermost
• Primary constituents of chorion are proteins that undergo hardening process either
before or shortly after oviposition.
• Hardening is due to peroxidase-catalyzed protein cross linking. In some species
chorion is sub divided into two anatomically distinct layer called endochorion and
exochorion
• Endochorion is a homogeneous electron dense layer, while the exochorion consists of a
sperm.

5. TYPES OF EGGS
• A) Singly laid eggs
• 1.Sculptured eggs: eg; castor butterfly.
2.Elongate egg: eg. sorghum shoot fly.
3.Rounded egg: eg. citrus butterfly .
4.Nit: egg of head louse is called nit.
5.Egg with float: Eg. Anapheles mosquito
• B) Eggs laid in groups:
1.pedicilate eggs: Eg Green lacewing
2.Barrel shaped: eggs are barrel shaped eg. Stink bug.
3.Ootheca: eg.cockroach
4.Egg pod : eg.Grasshopper
5.Egg mass: Eg.Rice stem borer
6.Egg raft :eg .culex
7.Egg case: eg.Mantid

6. EARLY EMBRYONIC DEVELOPMENT
• The early embryonic development is stated immediately after fertilization of the eggs by
sperms and it is occurred through a series of events that include
• Fertilization and zygote formation
• Cleavage.
• Formation of blastoderm
• Formation of vitellophages
• Formation of germband.
• Gastrulation
• Formation of embryonic membrane
• Blastokinesis
• Formation of appendages
• Formation of organ system

7. Fertilization and zygote formation
• Sperms released from the spermatheca of the females pass through
the micropyle. Usually egg nucleus is diploid until the entry of sperm
stimulates meiotic division leading to the formation of one nucleus and
three polar bodies ( which are eventually reabsorbed into the yolk)
Cleavage
• Cleavage is the repeated mitotic divisions of fertilized ovum zygote .
zygote divisions produce large number's of nuclei lacking cell
membranes but each surrounded by a small island of cytoplasm is
called energid.

8. Formation of Blastoderm
• The energids move and migrate towards the periphery (periplasm) of the egg and
arrange in a layer of circle within the yolk. The energids may undergo further, one
or more mitotic divisions and retain the distinct cell walls and subsequently form a
layer of cells, called the blastoderm. The blastoderm, in true sense, is the primary
germinal epithelium.it lies just beneath the vitelline membrane.
• Formation of vitellophages:
• All energids do not migrate to the periphery to form the blastoderm some of them
lie behind within the yolk are called the yolk cells, merocytes or vitellophages.
Vitellophages digest(liquify) the yolk and make nutrients available to the
developing embryo and also may form a part of the midgut epithelium.
vitellophages formation of blastoderm

9. Formation of germband
• Initially blastoderm forms a thin layer of cuboid cells all over the yolk.
Subsequently, they become thicker in the ventral region of the egg and become
columnar. This thickening is called embryonic primordia or germband which
develops future embryo. The rest blastoderm remains as extra-embryonic
membrane called serosa.
• Gastrulation:
• It is the process by which the mesoderm and endoderm are invaginated within
the ectoderm. The germ becomes differentiated into a median called middle
plates and two lateral areas called lateral plates. The gastrulation stage begins
when the mesoderm is formed from the middle in one of the three ways:
• 1.by an invagination of the middle plates (figure a)
• 2. by growing lateral plates over middle plate (figure b)
• 3.by proliferation of cells from the inner surface (figure c)

10. • Formation of embryonic membrane
• The germ band becomes covered by one or more
embryonic membranes. Soon after formation of
germ band, the serosa from either side extend until
both extensions meet and fuse in the ventral
midline. Small cavity forms on the ventral surfaces
of the germ band called amniotic cavity. The
amniotic cavity is bounded by a membrane is
called amnion.
• Blastokinesis:
• The embryo begins to move within the yolk,
undergoing rotation, revolutions and marked
displacement, the phenomenon is called
blastokinesis.
The movement taking place the posterior to
anterior pole of egg called anatrepsis.
whereas those from ventral to dorsal surface of the
egg are called katatrepsis

11. • Formation of Appendages
• With the differentiation of germ layers, the germ band begins the process of
metamerism the first division appears between the future labial and the first
thoracic segments, dividing the embryo into broad head region called
protocephalon and along narrow posterior trunk the protocorm. Soon after
the prethoracic as well as thoracic segmentation commence so that 3
segmented thoracic region is differentiated from the long unsegmented
protocorm- protopod phase. The unsegmented protocorm represents the
abdomen.
• Polypod stage: segmentation of abdomen and appendage formation occurs
and
• oligopod stage: in which last rudiments of abdomen form appendages.
• Concurrently with the formation of abdominal appendages a number of
ectodermal invaginations develop from which endoskeltal components,
various glands, the tracheal system, and certain parts of the reproductive
tract differentiate.

12. • Organogenesis:
• Organogenesis is the phase of embryonic development that starts at the end of
gastrulation and continues until the birth. During organogenesis, the three germ
layers formed from gastrulation: the ectoderm, endoderm and mesoderm form
the internal organs of the organism. Cells of each of the 3 germ layers undergo
differentiation, a process where less specialized cells become more specialized
through the expression of the specific set of genes.
• Ectoderm: outer most germ layer of the developing embryo forms the brain and
nervous system
• Mesoderm: middle germ layer of the embryo will form blood, heart, kidney,
muscles, and the connectives.
• Endoderm: innermost germ layer of the embryo gives rise to gastrointestinal
and respiratory organs, by forming epithelial linings, and organs such as liver,
lungs and pancreas
• GENERAL
SUMMERY OF INSECT EMBRYOGENESIS FROM FERTILIZATION
TO COMPLETE OF ORGANOGENESIS

13. • Post embryonic development.
• All that takes place after hatching or birth is post embryonic development
• Hatching: During the hatching process the young insect may chew its way through the eggs chorion
or it may swell in size by imbibing air until the eggshell “cracks” along a predetermined line of
weakness. Once the hatchling emerges, it is called a first instar nymph or larva. As it grows, it will
continue to develop and mature. These post embryonic changes are known as Morphogenesis.
• Once an insect hatches from the egg it is usually able to survive on its own, but it is small, wingless,
and sexually immature. Its primary role in life is to eat and grow. If it survives, it will periodically
outgrow and replace its exoskeleton (a process known as moulting). Collectively, all changes that
involve growth, moulting and maturation are known as Morphogenesis.
• Metamorphosis: it is defined as the series of changes through which an insect passes in its growth
from the egg through larva and pupa to adult, or from the egg through nymph to adult. These changes
are influenced by two hormones namely juvenile hormone and moulting hormone.
• Types of metamorphosis: 1. Ametabola development
2.Metabola development: 1. Hemimetabola
2.paurametabola
3.holometabola and 4. Hypermetabola

14. • Anamorphic development(anamorphosis)
• It is a progressive addition of abdominal segments during the development.
Anamorphosis occurs only in order prothura and no any other order of insect; however, it
is known to occur in some other arthropods.
• Ametabolous development (Epimorphosis)
• No metamorphosis, more primitive type, very little or no change during development,
young resemble adults, moulting occurs even during adulthood and fully developed
genetalia found in adults eg: Collembola (spring tails), thysanuran (silver fish)
dipluran(japygids)
• Hemimetabola (incomplete metamorphosis): is the mode of development of certain
insects that includes three distinct stages: the egg, naiad, and the adult stage, or imago.
These groups go through gradual changes; there is no pupal stage. Young one does not
resemble adults in all characters, habitat is different .eg: Dragonflies, damselflies,
mayflies.
• Paurametabola (Gradual metamorphosis): The young ones are called nymphs. They are
terrestrial and resemble the adults in general body form except the wings and external
genitalia. Their compound eyes and mouth parts are like that of adults. e.g., Cockroach,
grasshopper, bugs

15. • Holometabola (complete metamorphosis): is a form of insect development which
includes four life stages: egg, larva, pupa, and adult Holometabolism is a
synapomorphic trait of all insects in the superorder Endopterygota. Wing development
is internal, stages are structurally dissimilar, and habitat is different. egg: butterfly,
moth fly, bees
• Hypermetabola: Hypermetamorphosisis is exceptional in that some instars, usually
larval instars, are functionally and visibly distinct from the rest.eg. Blister beetle it
includes egg-triangulin-grub-pupa-adult.
• LARVA
• Immature stages of exopterygote insects are known as Nymphs and endopterygote
insects are known as Larvae.
• Types of larva.
• 1.Protopod larva: The larva emerges (hatch)from egg which is still in an early
embryonic phase as the egg contain less yolk material. The larvae are partially
developed. They possess well developed head and thoracic segments but lack
segmentation in the abdomen. They possess rudimentary cephalic and thoracic
appendages but no abdominal appendages. They have partially developed digestive
system and underdeveloped respiratory and nervous systems. Eg: endoparasitic
Hymenoptera

16. • 2. Oligopod larva: These are characterized by the presence of well-developed thoracic legs, head
capsule and without any abdominal legs. These are classified in to two types viz., campodeiform and
scarabeiform.
• a) Campodeiform: Resemble dipluran campodidae, thoracic legs are long and generally
predators. eg. Grub of antlion
• b) Scarabeiform: Body is ‘c’shaped, thoracic legs are short and sluggish, burrow into wood or soil. Eg.
Grub of rhinoceros beetle.
• 3. Polypod larva (Eruciform larva): The larva possesses well defined segmentation of the body with
three pairs of thoracic legs, 2-5 pairs of abdominal legs (3rd, 4th, 5th, 6th and 10th abdominal segment.
They are phytophagous and destructive
• Different types of polypod larvae:
• A. Hairy caterpillar Larval body is fully covered with hairs Eg: Redhairy caterpillar (Amsacta
albistriga), Castor hairy caterpillar ( Pericalia ricini).
• B. Sphingid caterpillar / larva the larva consists of a horn (or) hook on the dorsal surface of 8th
abdominal segment. Eg: Acherontia styx (Gingelly death’s head moth)
• C. Looper: Only two pairs of abdominal legs present on 6 th and last abdominal segment. During
walking the insect body forms a complete loop like structure hence, the name looper. Eg: Mango looper

17. • D. Semilooper: e.g.: Castor semilooper (Achoea janata). First two pairs of
abdominal legs (on 3 rd and 4 th segments) are reduced, hence a part of the insect
body forms a small loop during its movement Eg: Castor semilooper Achoea janata.
• 4. Apodous larva: These are characterized by the absence of trunk appendages (or)
legs. They possess 3 pairs of sensory papillae in the place of thoracic legs. They are
usually derived from Oligopod type. Based on the degree of development of the head
capsule and its appendages,these larva are divided in to 3 types.
• a. Eucephalous: e.g.: Sub order Nematocera of Diptera, Mosquito (Culicidae) The
larva consists of a well sclerotized head capsule.
• b. Hemicephalous. e.g.: Brachycera of Diptera, robberflies (Asilidae) Larva possess
partially developed head capsule.
• c. Acephalous e.g.: Cyclorrhapa of Diptera, Muscidae (houseflies) The larva are
characterized by the absence of head capsule and mouth parts are represented by
mouth hooks.
• PUPA
• It is resting, inactive stage of the holometabolous insects and transitional phase
during which the wings are developed and the insect attain matured sexual organs.
The pupa is incapable of feeding, locomotion except in some cases where they crawl
(Neuroptera) (Aphid lion), can swim e.g.: mosquitoes. Pupa represents a pharate adult
stage and later it emerges as adult, pupation may take place either in soil, or on the
plant surface or within the webs.

18. • Pupae is divided on the following bases.
• I. Based on the presence or absence of powerful mandibles.
• Decticous pupae: Possess relatively powerful mandibles which are used for escaping of the adult
from the cocoon i.e., to break the cocoon. e.g.: Neuroptera
• Adectious pupae: Do not possess the mandibles but with the help of other appendages, adults
escape from the cocoon eg: Lepidoptera, Diptera.
• II. Based on the attachment on the appendages (or) shape of the pupae.
• 1. Exarate pupa: e.g.: most of the Coleoptera The pupae have appendages which are free without
any secondary attachment to the body
• 2. Obtect pupa eg: Lepidoptera (moths) The pupae have appendages which are firmly pressed
against the body and the pupa is highly chitinized.
• 3. Coarctate: e.g.: Cyclorrhapha of Diptera (housefly) The pupa remain enclosed in a puparium
formed by the last larval skin and the pupa looks like a capsule or barrel.
• 4. Chrysalis: It is an obtect type of pupa which has golden colouration and a stalk eg: butterflies
• 5.Tumbler: Obtect type, comma shaped with rudimentary appendages breathing trumpets present
in the cephalic end, anal paddles present at the end of the abdomen. jerky movements are
produced by the anal paddles. very active pupa.eg. pupa of mosquito

19. MODIFICATION IN DIFFERENT ORDER:
• Order: thysanura
• Embryonic:Thysanura do not copulate but transfer sperm indirectly after a more or less complicated courtship. The male
spins a thread or threads with the aedeagus and deposits sperm droplets on them; the female then encounters the thread and
takes up the sperm into her reproductive tract. The absence or rarity of males in some species suggests that parthenogenesis
may occur. the eggs of Thysanura are relatively large, somewhat variable in shape and the number laid in one season does not
exceed about 30.
• Post embryonic:Hatching takes place with the aid of a cephalic spine and the early instars may lack scales and styles on the
coxae and abdomen. External genital rudiments first become apparent at the fourth instar in Petrobius brevistylis and the
eighth in Ctenolepisma longicaudata, and sexual maturity is reached after a further five or six moults. Moulting continues
into the adult stage and the total number of moults may be considerable - from 25 to 66 have been recorded in Ctenolepisma
and from 19 to 58 in Thermobia. At each adult moult the cuticular lining of the spermatheca is lost, together with its contents,
so that copulation has to occur in each adult ins tar in order that fertile eggs may continued to be laid.
• Order: Diplura
• Embryonic:sperm transfer has been observed only in the Campodeidae where it is indirect; the male deposits stalked
spermatophores from which the female takes up the terminal sperm droplet without courtship. The eggs of Campodeidae and
Japygidae are normally laid in a mass of up to 40, suspended in a cavity in the soil on a common stalk. in Parajapyx,
however, each egg is individually stalked.
• Post embryonic:The first instar is a short-lived, incompletely formed 'prelarva' and it and the more normal second ins tar live
on the yolk in their gut and. Simple maternal care of the aggregated young has been described in the Japygidae. In Campodea
the individuals live for 2-3 years, moulting up to 20 times a year and attaining sexual maturity after 8-11 moults. Japygids
may have fewer instar, but they also moult after reaching the adult state.

20. • Order:Odonata
• Oviposition in dragonflies may be either endophytic or exophytic. In the latter case the eggs are rounded and are either dropped
freely into the water or attached superficially to aquatic plants. This method is the rule among the Anisoptera. In Sympetrum
and Tetragoneuria the eggs are laid in gelatinous strings attached to submerged twigs. Endophytic oviposition is characteristic of
the Zygoptera and the Anisopteran families Aeshnidae and Petaluridae. Pressure of the head of the embryo against the chorion is
the immediate cause of hatching since, it forces open the lid-like anterior extremity of the egg.
• Post embryonic development.
• Hemimetabolic metamorphosis is seen with egg- naiad- adult. The number of instars varies between ten to fifteen. The newly
hatched insect is known as the pronymph. The insect which emerges is in its second instar and is now a free nymph fully
equipped for its future life. The nymphs of the Odonata are campodeiform, the immature stages of the Odonata are exclusively
aquatic, living in various situations in fresh water. During the metamorphosis increase in the size of the compound eyes, The
antennal segments increase in number, and the wing-rudiments change so that the developing hind wings overlap the anterior
pair; the wing-bearing segments increase in size. The internal changes which occur in the fully grown nymph just before
metamorphosis is complete.
• Order: Orthoptera
• Embryonic: short germband came from panoistic overies. type of eggs they laid are called Egg pod. In eggs of this group
insects the periplasm at the posterior pole of the egg receives energids first.11 pairs of abdominal appandages evaginate before
provisional dorsal closure.
• Postembryonic development: Gradual metamorphosis takes place. The first ins tar in most, if not all, Orthoptera, is the so-
called 'vermiform larva' (pronymph) with a loose cuticle which envelops the appendages in such a way that they are pressed to
the sides of the body and their segmentation is indistinct. In many Orthoptera the soft cervical membrane plays an important
part during hatching: it is capable of being distended, by the influx of blood, into a swollen dorsal ampulla which protrudes
immediately behind the head. The wing-pads usually appear in the third instar. In Melanoplus and Oecanthus, which pass
through six nymphal instars.

21. • Order: Dictyoptera:
• Embryonic: Cockroaches mate after a relatively simple courtship and a spermatophore is formed. In the of dictyoptera
growth of germ band may occur on on the surface of the yolk. The type eggs is ootheca. the ootheca is divided into two
rows of pockets by a longitudinal partition and each egg (16 in Blatta, 40 in Blattella) occupies a pocket. When the
nymphs are ready to emerge, the ootheca splits along its dorsal edge, the two halves separate and the young struggle out.
Three principal modes of reproduction have been distinguished, (i) Oviparity, (ii) Ovoviviparity, (iii) Viviparity, confined
to Diploptera punctata The eggs of mantids are laid in oothecae, which are attached to twigs, bark, walls and other objects.
Each female makes a number of these cases (twenty-two have been recorded in Miomantis and their type of construction
varies according to the species. the ootheca consists of a frothy secretion which hardens into a firm, spongy substance.
• Post embryonic: Hemimetabolic metamorphosis, The first-stage nymph in Blatta orientalis is a 'pronymph' with the ins
tar of very short duration and its appendages not fully formed and apparently soldered down to give it a vermiform
appearance. The six subsequent nymphal instars of this species can be distinguished from one another by differences in the
developing genital segments, number of cercal segments and size of wing-pads and take 279 days to complete their
development at 27· 5° C.
• Order:Thysonoptera
• Embryonic: The eggs of the Terebrantia are more or less reniform, while those of the Tubulifera are commonly elongate
oval. In the first suborder the female cuts a slit with her saw-like ovipositor, laying the eggs singly in the tissues of the
host plant. The Tubulifera lay their eggs externally, either singly or in groups, upon leaves, stems, under bark, etc.
• Post embryonic: members of this Exopterygote group are clearly holometabolous and have presumably evolved the
condition quite independently of the Endopterygote insects. The prepupa and pupa resemble the Endopterygote pupa.
prepupa and pupa have conspicuous wingpads.

22. • Order: Hemiptera(bugs):
• Embryonic: Almost all Heteroptera are oviparous, The eggs of the Heteroptera show
considerable diversity of form and chorionic structure, and they vary in the mechanisms of
respiration, fertilization and eclosion.
• Postembryonic development: is gradual in most Hemiptera, but colour changes are often very
marked. The most sharply pronounced modifications are concentrated in the last ecdysis from
the final immature instar to the imago. The external morphological changes during development
involve the segments of the antennae and tarsi, the latter frequently not attaining their full
number until the adult stage. The shape of the head and thoracic segments, more particularly the
pronotum, undergo marked changes in different instars. Wing-rudiments are small or scarcely
distinguishable in the 3rd instar but are evident in the 4th
• Order: coleoptera
• The eggs of Coleoptera are usually ovoid in form, and rarely exhibit any marked diversity of
form of structure as is seen. while in the Meloidae they are small, and the number laid by a
single female may run into several thousand. the Hydrophilidae enclose them in cocoons, while
among Cassidinae they are protected by highly specialized oothecae.
• Post embryonic: Holometabolic metamorphosis(complete) , includes egg-larva-pupa-adult
stages. The primitive form of larva is campodeiform and other have scarabeform and apod type
larvae. Most of this group have exarate type of pupa.

23. • Order: Diptera
• Embryonic: Dipteran eggs often have a simple external structure, particularly if they develop in
permanently wet habitats. Some eggs, notably those of some Culicids, are organized into groups, called
egg-rafts in this family.
• Post embryonic: holometabolic metamorphosis(complete), includes egg-larva-pupa-adult. Here larvae
called maggots. This group insects have apod/vermiform of larvae (hemicephalous, eucephalous,
acephalous) and pupa is coarctated pupa, mosquito has tumbler(obtect) pupa, which is very active
pupa.
• Order: Lepidoptera
• Embryonic: The eggs of Lepidoptera are roughly divisible into two forms: (I) ovoid or flattened, with
the long axis horizontal. (2) upright and either fusiform, spherical, or hemispherical, with the axes
either equal, or the vertical axis the longest. The micropyle is usually placed in a slight depression at
one extremity of the horizontal axis of an ovoid type of egg, and at the summit in the upright form. The
average number of eggs laid by many species is high, sometimes exceeding 1000 (Noctua fimbriate,
Zeuzera pyrina), and they are deposited in a great variety of ways and positions.
• Post embryonic: Holometabolic metamorphosis(complete), includes egg -larvae -pupa-adult. Larvae
is called caterpillar. larvae have different modification polypod larvae (hairy caterpillar, slug,
caterpillar, semilooper, looper). Pupa includes obtect pupa(chrysalis)t, this group has prolegs along
with thoracic legs.
•

24. • CONCLUSION:
• Most insects are oviparous and lay eggs with a pre-packaged source nutrients -yolk-that is
surrounded by a rigid eggshell with features important for fertilization, respiration and water
regulation .Embryogenesis begins with the formation of zygote nucleus and ends with development
and hatching of a nymph or larva. The extraordinary series of events takes place during
embryogenesis.
• Postembryonic development is divided into a series of stages , each separated from the next by
molt.in hemimetabolous insects the developmental changes that transform a larva into the adult are
relatively slight; in holometabolous insects the changes are marked and a pupal stage is
interpolated between the final larval stage and the adult.